sunlight-driven membrane-supported photoelectrochemical water...
TRANSCRIPT
Membrane-Supported Nanorod PEC H2 Production
SUNLIGHT-DRIVEN MEMBRANE-SUPPORTED PHOTOELECTROCHEMICAL WATER
SPLITTING
Nathan S. Lewis, Harry A. AtwaterHarry B. Gray, Bruce S. Brunschwig
Jim Maiolo, Kate Plass, Josh Spurgeon, Brendan Kayes, Mike Filler, Mike Kelzenberg, Morgan Putnam
California Institute of TechnologyPasadena, CA 91125
LightFuel
Electricity
Photosynthesis
Fuels Electricity
Photovoltaics
sc
e
SC
CO
Sugar
H O
O
2
2
2
Energy Conversion Strategies
Semiconductor/LiquidJunctions
H2
O
O H22
SC
Fuel from Sunlight
QuickTime™ and aYUV420 codec decompressor
are needed to see this picture.
Photoelectrochemical
Cell
metal
e-
e-
O2
H2
O
H2
H2
O
e -
h+
Light is Converted to Electrical+Chemical Energy
LiquidSolid
SrTiO3
Membrane-Supported Nanorod PEC H2 Production
FTO slide printed with binary combinations of eight metals, pyrolyzed
to form mixed metal oxides.
Typical photocurrent map of a slide of metal oxides. The X-
and Z-coordinates give each spot’s identity, which is correlated with photo-catalytic activity. Here, combinations of Zn and Co in row #9 show photoanodic
current
Combinatorial Discovery of Nanorod MaterialsCombinatorial Discovery of Nanorod Materials
Membrane-Supported Nanorod PEC H2 Production
Lessons from PhotosynthesisLessons from Photosynthesis
Membrane-Supported Nanorod PEC H2 Production
Turner CellTurner Cell
Membrane-Supported Nanorod PEC H2 Production
2H2
O2
catalystanode
H2
catalystcathodeSolar PV & membrane
H2
O
O24H+
System ConceptSystem Concept
Membrane-Supported Nanorod PEC H2 Production
Why Radial Junction Solar Cells?Why Radial Junction Solar Cells?
•
Traditional device design requires long minority carrier diffusion lengths, and therefore expensive materials
p-type
n-type
hν
Traditional
single junction solar cell
Idealized radial junction
nanorod solar cell
~Ln
1/α
•
Radial geometry allows for high quantum efficiency despite short
minority carrier diffusion lengths
~Ln
Membrane-Supported Nanorod PEC H2 Production
Device ModelingDevice Modeling
Comparison of photovoltaic efficiency for (a) planar and (b) nanowire Si devices as afunction of absorber thickness and minority carrier diffusion length
B.M. Kayes, H. A. Atwater, and N. S. Lewis, J. Appl. Phys. 97
114302 (2005)
Relatively high efficiencies possible despite low diffusion lengths if depletion region recombination can be minimized
Membrane-Supported Nanorod PEC H2 Production
Structural Organization in NatureStructural Organization in Nature
RootForest
Stomata Leaf
Membrane-Supported Nanorod PEC H2 Production
Enables New MaterialsEnables New Materials•
High efficiencies possible despite low diffusion lengths=> Inexpensive materials
–
Cheaper traditional materials•
Silicon as a model system
–
Non-traditional, earth-abundant materials•
Tandem partners with silicon
Membrane-Supported Nanorod PEC H2 Production
Relaxes Catalyst Activity RequirementsRelaxes Catalyst Activity Requirements
Hydrogen Evolution Reaction
Membrane-Supported Nanorod PEC H2 Production
Why Macroporous Silicon?Why Macroporous Silicon?
•
Macroporous
silicon can be made from single-crystalline (100) silicon starting material.
•
Bulk-limited VOC can be calculated from the Shockley diode equation:
⎟⎟⎠
⎞⎜⎜⎝
⎛=
0
lnJJ
qkTV ph
OC
Idealized radial junction
nanorod solar cell
Idealized macroporous
silicon model sample
Dp
ip
NLnqD
J2
0 =
Membrane-Supported Nanorod PEC H2 Production
Making MacroporesMaking Macropores
5% HF + 10 mM SDS
Membrane-Supported Nanorod PEC H2 Production
Macroporous SiliconMacroporous Silicon
Membrane-Supported Nanorod PEC H2 Production
I-V MeasurementsI-V Measurements
Membrane-Supported Nanorod PEC H2 Production
I-V Curves – 100 mW/cm2I-V Curves – 100 mW/cm2
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are needed to see this picture.
Membrane-Supported Nanorod PEC H2 Production
Testing a Nanorod Array Solar CellTesting a Nanorod Array Solar Cell
•
Characterize a working nanorod
solar cell–
Ready fabrication of nanorod
electrodes to put into
a working cell •
CdSe, CdTe
–
While not an ideal commercial material (toxicity, abundance), CdSex
Te1-x•
is diffusion length limited
•
has ideal bandgap
(~1.4 eV) •
allows for an early test of the theory
Membrane-Supported Nanorod PEC H2 Production
The Alumina TemplateThe Alumina Template
• Allows for dense, uniform array of rods
• Carefully controlled dimensions
• Easily removable template
Membrane-Supported Nanorod PEC H2 Production
The Alumina TemplateThe Alumina Template
• Allows for dense, uniform array of rods
• Carefully controlled dimensions
• Easily removable template
Membrane-Supported Nanorod PEC H2 Production
Porous Alumina
CdSe
layer to prevent shunting (via sputtering)Titanium Back Contact (via sputtering)
CdSeTe
Nanorods(via electrodeposition)
Alumina template removed in 1 M NaOH
CdSeTe NanorodsCdSeTe Nanorods
Membrane-Supported Nanorod PEC H2 Production
•
Excellent control over the dimensions and composition of these rods is possible
CdSeTe NanorodsCdSeTe Nanorods
Membrane-Supported Nanorod PEC H2 Production
•
Very uniform over relatively large area (a few cm2)
CdSeTe NanorodsCdSeTe Nanorods
Membrane-Supported Nanorod PEC H2 Production
CdSeTe NanorodsCdSeTe Nanorods•
Use a liquid junction to make a photoelectrochemical
cell•
Measure current-voltage properties in the dark and light and compare nanorod
performance to an analogous planar electrode
•
Thin sputtered CdSe
layer to prevent shunting through back contact –
negligible contribution to performance
Membrane-Supported Nanorod PEC H2 Production
Why Radial Junction Solar Cells?Why Radial Junction Solar Cells?
•
Traditional device design requires long minority carrier diffusion lengths, and therefore expensive materials
p-type
n-type
hν
Traditional
single junction solar cell
Idealized radial junction
nanorod solar cell
~Ln
1/α
•
Radial geometry allows for high quantum efficiency despite short
minority carrier diffusion lengths
~Ln
Membrane-Supported Nanorod PEC H2 Production
CdSeTe NanorodsCdSeTe Nanorods•
Spectral response:
0
0.02
0.04
0.06
0.08
0.1
0.12
400 500 600 700 800 900 1000
External Quantum Yield
Wavelength (nm)
PlanarNanorod
•
As expected from IV curves, planar usually exhibits better quantum yields –
the above curves were chosen because their similar magnitudes allow for ease of shape comparison
•
Shape of the curve is more significant •
Nanorod
samples lose less quantum yield
in the red•
Since longer wavelength light penetrates deeper into the semiconductor, this is an indication that the nanorod
geometry is more effectively collecting the carriers
Membrane-Supported Nanorod PEC H2 Production
•
Nanorod
Array Growth Procedure
Porous Alumina
Conductive Catalyst Metal (via evaporation)
Mounting Wax
Supportive Nickel Substrate (via electrodeposition)
Silicon Nanorods
Alumina template removed in 1 M NaOH
Si NanorodsSi Nanorods
Membrane-Supported Nanorod PEC H2 Production
•
Silicon nanorod
array after template is removed
Si NanorodsSi Nanorods
Membrane-Supported Nanorod PEC H2 Production
Spatially resolved photocurrentSpatially resolved photocurrent
NSOM schematic SEM-NSOM Topography
Overlay
4-Point Probe
5 μm
10 μm
Al/Ag
Rwire
= 17.9 MΩ
RC
= 200 kΩ
ND
~ 5×1013
cm-3
λ=488 nm
•
Near-field scanning optical microscopy (NSOM) can measure nanowire photoconductivity as a function of excitation location.
•
Carrier collection lengths can be studied as a function of nanowire diameter, growth conditions, passivation, etc...
(1.18 μm diameter)
5 μm
Membrane-Supported Nanorod PEC H2 Production
Effective diffusion lengthEffective diffusion length
Charge continuity for electrons with effective, single-τ
recombination rate:
NSOM suggests minority carrier collection length is comparable to wire diameter.
10 μm
Pho
tocu
rrent
(pA
)
-200
200
Pho
tocu
rrent
(nor
m.) 1
-10 µm 10 µm
Charge transport by drift and diffusion:
( ) ( ) ( ) nn n n dxJ x q n x E x qD δμ ∂= +
Monitor terminal current as function of injection point:
0 0| |meas n x p xJ J J= == +
Ln,eff = = 1.9 µm ↔
τn,eff ≈
1 ns
Lp,eff = = 2.2 µm ↔
τp,eff ≈
4 ns
,n n effD τ
,p p effD τ
Al
200 mVbias
, |injn p x x phg I= =when
200 mVbias
neffn
n gnxJ
qtn
+∂
−∂∂
=∂∂
,
1τδδ
Vary τ
to match measured photocurrent profile:Wire position
Energy band diagram
Incr
easi
ng e
nerg
y
Membrane-Supported Nanorod PEC H2 Production
•
Low Trap State Density•
Stable in Ambient Conditions
•
Facile Interfacial Charge Transfer
Surface Requirements for Semiconductor Heterojunctions
Surface Requirements for Semiconductor Heterojunctions
hν
Traditional planar photovoltaic
~L
Idealized high aspectratio photovoltaic
Membrane-Supported Nanorod PEC H2 Production
Oxide Buffer Layer - Pattern FidelityOxide Buffer Layer - Pattern Fidelity
100 μm100 μm
10 μm 10 μm
H2
anneal
1000oC
No
oxid
e bu
ffer l
ayer
H2
anneal
1000oC
Oxi
de b
uffe
r lay
er
50 μm10 μm
An oxide buffer layer is critical for maintaining pattern fidelity during growth.
3 μm array, 500 nm Au, Tgrowth
= 1000oC, Pgrowth
= 760 Torr
Membrane-Supported Nanorod PEC H2 Production
Large Area Au-Catalyzed Si ArraysLarge Area Au-Catalyzed Si Arrays
100 μm
3 μm array, 500 nm Au, Tgrowth
= 1000oC, Pgrowth
= 760 Torr, 30 min growth, 2 mole % SiCl4
in H2
Membrane-Supported Nanorod PEC H2 Production
Large Area Au-Catalyzed Si ArraysLarge Area Au-Catalyzed Si Arrays
30 μm
Nearly 100% vertically aligned, 75 μm length microwire
arrays over areas > 1 cm2.
3 μm array, 500 nm Au, Tgrowth
= 1000oC, Pgrowth
= 760 Torr, 30 min growth, 2 mole % SiCl4
in H2
Membrane-Supported Nanorod PEC H2 Production
Copper-Catalyzed Si Wire ArraysCopper-Catalyzed Si Wire Arrays
250 μm
15 μm
Copper produces wire arrays that are structurally equivalent to gold.
3 μm array, 500 nm Cu, Tgrowth
= 1000oC, Pgrowth
= 760 Torr, 10 min growth, 2 mole % SiCl4
in H2
Membrane-Supported Nanorod PEC H2 Production
Si Wire Array JunctionsSi Wire Array Junctions
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are needed to see this picture.
Membrane-Supported Nanorod PEC H2 Production
•
Low Trap State Density•
Stable in Ambient Conditions
•
Facile Interfacial Charge Transfer
Surface Requirements for Semiconductor Heterojunctions
Surface Requirements for Semiconductor Heterojunctions
hν
Traditional planar photovoltaic
~L
Idealized high aspectratio photovoltaic
Membrane-Supported Nanorod PEC H2 Production
PCl5benzoyl
peroxide
or Cl2(g)
CH3
MgCl
Surface PassivationSurface Passivation
SiH
SiCl
SiCH3
Si(111)-CH3
0.38 nm
1 nm
Measured through STM,
T = 4 K
Membrane-Supported Nanorod PEC H2 Production
SEM of Chlorinated Si Nanorod ArraysSEM of Chlorinated Si Nanorod Arrays•
Chlorination with PCl5 is known to cause etching of silicon surfaces
Chlorinated 45 min
Chlorinated 10 min, then methylated
•
Functionalized surfaces appear rough, but etching is not destructive1 μm
2 μm
Membrane-Supported Nanorod PEC H2 Production
Electrical Performance of Methylated Silicon Nanowires
Electrical Performance of Methylated Silicon Nanowires
•
Methylated
wires (60 nm diameter)–
Sensitive to gate voltage –
Maintain higher conductance•
Hydrogen terminated surfaces–
Deteriorate
Haick, H.; Hurley, P. T.; Hochbaum, A. I.; Yang, P.; Lewis, N. S. J. Am. Chem. Soc. 2006, 128, 8990-8991.
Nor
mal
ized
ave
rage
co
nduc
tanc
e
Ave
rage
intri
nsic
co
nduc
tanc
e
Gate voltage (V) Time (h)
Membrane-Supported Nanorod PEC H2 Production
Constructing the Pieces of a Solar H2 Fuel Generator
Constructing the Pieces of a Solar H2 Fuel Generator
Polymer Embedding of Si Rod ArraysPolymer Embedding of Si Rod Arrays
Si*O
*
PDMS (polydimethylsiloxane)
Large Area Rod Array RemovalLarge Area Rod Array Removal
•
Large area arrays (> 1 cm2) transferred in one piece.
•
Conformal coating from top to bottom of rods
115 μm
Top-down view Side view
Embedded Rod Pattern FidelityEmbedded Rod Pattern Fidelity
Rod diameter
(μm)
Shortest inter-rod distance
(μm)
Angle (θ, °)
Before removal
2.5 ±
2 8.6 ±
2 92 ±
9
After removal
4.6 ±
4 8.8 ±
3 94 ±
2
•
Inter-rod angle and spacing is constant before and after PDMS casting and removal
•
Apparent rod diameter differences due to examination of rod tips versus base
θ
Large Area Rod Array RemovalLarge Area Rod Array Removal
•
Large area arrays (> 1 cm2) transferred in one piece.
•
Conformal coating from top to bottom of rods
115 μm
Top-down view Side view
Membrane-Supported Nanorod PEC H2 Production
Regrown ArraysRegrown Arrays
Polymerization from Si Rod SurfacesPolymerization from Si Rod Surfaces
Allylated
Si
base Methylated
Si base
SiCH2
Ru*n
SiCH2
Ru*H
1st
generation Grubbscatalyst
SiCH2H
H H
polymer sheath grown around rods
Membrane-Supported Nanorod PEC H2 Production
•
1.23 V needed to electrolyze water
•
Requires a heterojunction or dual junction
•
Single absorber: band gap of 2.0-2.6 eV
that straddles the necessary potentials
•
Photoanode and photocathode absorbers:
Band gaps can better match the solar spectrum (1.1-1.4 eV)
Allows for greater flexibility in materials design
Efficiency could approach 25%
•
Optimal structure would have several light-
absorbing layers absorbing different portions
•
of the spectrum with an overall potential greater than 1.23 V
Dual Junction Nanorod ArraysDual Junction Nanorod Arrays
Membrane-Supported Nanorod PEC H2 Production
Fe2 O3 NanowiresFe2 O3 Nanowires
Membrane-Supported Nanorod PEC H2 Production
2H2
O2
catalystanode
H2
catalystcathodeSolar PV & membrane
H2
O
O24H+
System ConceptSystem Concept
Membrane-Supported Nanorod PEC H2 Production
ConclusionsConclusions•
Without massive
quantities (10-20 TW by 2050) of clean energy, CO2
levels will continue to rise
•
The only sufficient supply-side cards we have are “clean”
coal, nuclear fission (with a closed fuel cycle), and/or cheap solar fuel
•
We need to pursue globally scalable systems that can efficiently
and cost-
effectively capture, convert, and store sunlight in the form of chemical fuels
–
He that can not store, will not have power after four
•
Semiconductor/liquid junctions offer the only proven method for achieving this goal, but we have a great deal of fundamental science to learn to enable the underpinnings of a cost-effective, deployable technology
–
Nanorods, randomly ordered junctions to generate the needed potential
–
Catalysts to convert the incipient electrons into fuels by rearranging the chemical bonds of water (and CO2
) into O2
and a reduced fuel
D
Si M
e
A
I-
I3-
ACN
hυ
Loade-
e-e-
e-
D
Si M
e
A
EtOH
H2
O
Four Strategies
New RedoxCouples
NonaqueousSolvents
SurfaceModification
InexpensiveSemiconductor s
S2-
CdS M
e
S22-